just guesses, because they are so infrequently worn these days, is that it somehow prevents eyeball deformation due to its rigidity and so the axial elongation does not occur.

I'm a little sceptical about that theory, but my own rx did not go increase much when I wore RGPs. After that I did have some progression, but also a million other things changed, so who knows.

RGPs however, are quite distinct in that they give absolutely perfect vision. better than soft contacts by a mile. Maybe some refractive thing as well, optometrist told me numerous times under-correction would lead to further progression.

Ortho-K is popular in Asia for prevention, those are rigid lenses worn at night. Deforms cornea a little, so it's like a reversible lasik thing. So, in this case, not only its rigid lenses, they also have weird geometry. Many report significant success.